Such a method is employed, for instance, for detecting raindrops present on the windshield of a motor vehicle to control a wiper motor. Such a device is accordingly referred to as a rain sensor. Such a method as well as such a rain sensor is known from U.S. Pat. No. 4,867,561. The detection method described in this document operates according to the reflection principle, utilizing total reflection on the outside of the windshield. To this end, the rain sensor comprises an infrared diode (IR diode) constructed as an illumination apparatus, the emitted beams of which are directed from the inside onto a section of the windshield to be observed. As a pickup unit, an optical sensor array which is preceded by a lens for imaging [light from] the IR diode onto its photosensitive surface is used. The illumination apparatus and the pickup unit are positioned with respect to one another such that light beams emitted from the light-emitting diode that are reflected by total reflection of the windshield are imaged onto the sensor array in case of a clean windshield. The previously known method uses the fact that said total reflection of the light beams emitted from the illumination apparatus occurs from the outside of the windshield to the sensor array in case of the absence of objects on the windshield. On the other hand, if raindrops are present on the outside of the windshield, light beams are output coupled by the raindrops, so that only a part of the light beams is reflected to the sensor array. Consequently, the light intensity that can be detected by the sensor array in case of the presence of objects on the windshield is lower compared to the light intensity detected for total reflection.
The signals of the sensor array are first filtered in an evaluation unit downstream of the sensor array in order that those signal components which cannot be associated with the light beams emitted from the illumination apparatus are not further evaluated. To this end, the filtering is designed to be wavelength-specific. The filtered sensor array output signal is then fed to a comparator element in which a threshold value comparison of the detected light intensity to a specified threshold value is performed. If the detected light intensity is lower than the threshold value, a control signal, which is applied to a microcomputer for triggering the wiper motor, is present at the output of the comparator element.
Even if the method or device described in this document permits an improved detection over and against older ones, the method is nonetheless subject to error influences. Sunbeams or other light beams refracted on the windshield, whose refraction also produces infrared components which can then impinge on the sensor array, can be considered interfering factors. Since the evaluation of the light intensity detected in the sensor array is adapted by specification of the threshold value to the emitted light intensity of the illumination apparatus, such secondary influences can superimpose the output coupling of light beams by raindrops adhering to the windshield, so that the resulting evaluation outcome no longer corresponds to the actual conditions.
Furthermore, the previously known method is not suited to detect objects on the windshield for which an output coupling of light beams does not occur, such as dust or the like.